Certain undesirable physiological manifestations, such as acne vulgaris, seborrhea, female hirsutism, androgenic alopecia which includes female and male pattern baldness, and benign prostatic hyperplasia, are the result of hyperandrogenic stimulation caused by excessive accumulation of testosterone ("T") or similar androgenic hormones in the metabolic system. Androgenic alopecia is also known as androgenetic alopecia. Early attempts to provide a chemotherapeutic agent to counter the undesirable results of hyperandrogenicity resulted in the discovery of several steroidal antiandrogens having undesirable hormonal activities of their own. The estrogens, for example, not only counteract the effect of the androgens but have a feminizing effect as well. Non-steroidal antiandrogens have also been developed, for example, 4'-nitro-3'-trifluoromethyl-isobutyranilide. See Neri, et al., Endocrinol. 1972, 91 (2). However, these products, though devoid of hormonal effects, compete with all natural androgens for receptor sites, and hence have a tendency to feminize a male host or the male fetus of a female host and/or initiate feed-back effects which would cause hyperstimulation of the testes.
The principal mediator of androgenic activity in some target organs. e.g. the prostate, is 5.alpha.-dihydrotestosterone ("DHT"), formed locally in the target organ by the action of testosterone-5.alpha.-reductase. Inhibitors of testosterone-5.alpha.-reductase will serve to prevent or lessen symptoms of hyperandrogenic stimulation in these organs.
The enzyme 5.alpha.-reductase catalyzes the reduction of testosterone to the more potent androgen, dihydrotestosterone, as shown below: ##STR1##
Finasteride, (17.beta.-(N-tert-butylcarbamoyl)-3-oxo-4-aza-5.alpha.-androst-1-ene-3-one ) as shown below, is a potent inhibitor of the human prostate enzyme. ##STR2## Under the trade name PROSCAR.RTM., finasteride is known to be useful in the treatment of hyperandrogenic conditions; see eg. U.S. Pat. No. 4,760,071. Finasteride is currently prescribed for the treatment of benign prostatic hyperplasia (BPH), a condition afflicting to some degree the majority of men over age 55. Einasteride's utility in the treatment of androgenic alopecia and prostatic carcinoma is also disclosed in the following documents: EP 0 285,382, published Oct. 5, 1988; EP 0 285,383, published Oct. 5, 1988; Canadian Patent no. 1,302,277: and Canadian Patent no. 1,302,276.
There are two isozymes of 5.alpha.-reductase in humans. One isozyme (type 1) predominates in sebaceous glands of most regions of skin tissue and is relatively insensitive to finasteride; the other (type 2) predominates in the prostate and is potently inhibited by finasteride.
In clinical trials, the efficacy of finasteride far exceeded expectations based on its perceived potency against the human prostate enzyme, for which finasteride was first thought to be a simple, rapidly-reversible inhibitor with K.sub.i =26 nM. For instance, circulating concentrations of finasteride comparable to this Ki actually reduced levels of dihydrotestosterone to values approaching those found in individuals genetically deficient in the prostate isozyme, and as long as two weeks were required for dihydrotestosterone to return to basal levels after withdrawal of finasteride (Stoner, J. Steroid. Biochem. Molec. Biol. 37: 375-378 (1990) and Gormley et al., J. Clin. Endocrinol. Metabol. 70: 1136-1141 (1990)). A closer evaluation of the interaction of finasteride with the human prostate (type 2) isozyme led to appreciation that finasteride and certain analogs thereof are slow-binding inhibitors, such that their potency had been mistakenly underrated in standard fixed-time assays (Harris et al., Proc. Natl. Acad. Sci. U.S.A., 89: 10787-10791 (1992)). Independently, Faller et al., also recognized the inconsistency. Faller et al., have recently described in detail the slow-binding behavior of finasteride, reaching the conclusion that finasteride binds to the human prostate isozyme with a rate constant of 2.7.times.10.sup.5 M.sup.-1 s.sup.-1 to form an essentially irreversible enzyme-inhibitor complex with a K.sub.i &lt;&lt;1 nM (Faller et al., Biochemistry 32: 5705-5710 (1993)).
Although finasteride is not a significant inhibitor of human skin (type 1) isozyme at doses employed in the treatment of BPH, finasteride does slowly form a comparable high affinity complex with this isozyme. As determined by Tian, et al., Biochemistry 33: 2291-2296 (1994), the second-order rate constant for formation of this complex is 4.0.times.10.sup.3 M.sup.-1 s.sup.-1, which is about 1% of the rate constant against the prostate isozyme. Based on the apparent irreversible inhibition and on structure-activity considerations, Tian et al., proposed that finasteride binds to the enzyme covalently as a Michael acceptor. U.S. Ser. No. (08/369,513), filed Jan. 4, 1995, describes that finasteride and other 3-oxo-4-aza steroids having a 1,2 double bond are mechanism-based irreversible inhibitors of 5.alpha.-reductase.
The present invention demonstrates, in contrast to the expectation in the art, that 3-oxo-4-oxa and 4-thiasteroids having a 1,2 double bond are novel mechanism-based irreversible inhibitors of 5.alpha.-reductase. These 3-oxo-4-oxa and 4-thiasteroids having a 1,2-double bond are recognized as a substrate by the human 5.alpha.-reductase type 1 and type 2 enzymes, and in the course of the enzymatic reduction, the 3-oxo-4-oxa and 4-thiasteroid having a 1,2-double bond forms a covalent adduct with the pyridine-nucleotide cofactor (NADPH). The covalent inhibitor-cofactor complex formed between the 3-oxo-4-oxasteroid anion and the oxidized nicotinamide cofactor is bound by the enzyme as a potent collected-product inhibitor.
Kaplan et al. (Everse et al., Bioorganic Chem. 1: 207-233 (1971)) in their work with pyridine nucleotide-linked dehydrogenases identified analogous high-affinity, abortive ternary complexes formed spontaneously (in the reverse direction) by several pyridine nucleotide-linked dehydrogenases, notably lactate dehydrogenase. Lactate dehydrogenase mistakenly adds pyruvate to NAD.sup.+ to form the inhibitor shown below: ##STR3## wherein B.sup.- represents a basic group on the enzyme active site and ADPR represents adenosine-diphospho-ribose.